Gregorian calendar

The Gregorian calendar is the calendar established under the auspices
of Pope Gregory XIII in 1582 to replace the Julian
calendar. Correcting at a stroke the 10-day accumulated error of the
Julian calendar, the main innovation was that century-years were discounted
as leap years unless they were divisible
by 400. The Gregorian calendar is now used as the civil calendar in most
countries. Historical or astronomical dates based on the Gregorian calendar
are said to be New Style dates as distinct from Old
Style dates that refer to the Julian calendar.